Because legendary horns, other than the Happy Horn, break down into a horn relay. If they put horns in the kiosk, you'd be able to get more than one, break them down, and get horn relays WITHOUT HAVING TO PAY FOR THEM. This. Cannot. Be. Why don't they just change that, so that any horn you get from the kiosk has the same properties as the Happy Horn? Because their development tools are garbage and they don't have the resources to spend on something like this. Maybe in Destiny 2. Maybe.

They should make any you get from the kiosk not break down into relays. Or completely remove relays altogether